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Miranda 天卫五(中英)

2006-02-21 00:00

  Miranda is the eleventh of Uranus's known satellites. Miranda is the innermost of Uranus' large moons.

  orbit:    129,850 km from Uranus

  diameter: 472 km

  mass:     6.3e19 kg

  Miranda is a daughter of the magician Prospero in Shakespeare's The Tempest.

  Discovered by Kuiper in 1948.

  Voyager 2 was forced to fly close to Uranus in order to get the boost it needed to go on to Neptune and due to the orientation of the whole system at almost right angles to the ecliptic only Miranda was approached closely. Before Voyager, of course, little was known about Miranda and as it is not the largest or in any other way remarkable, it probably would not have been chosen as the prime target at Uranus. Voyager's good luck held up, however, as Miranda turned out to be by far the most interesting.

  Miranda is about half water ice and half rocky material.

  Miranda's surface is all mixed up with heavily cratered terrain intermixed with weird grooves, valleys and cliffs (one over 5 kilometers high).

  At first, Voyager 2's images of Miranda were a mystery. Everyone had expected  that Uranus' moons would show very little history of internal activity (like Callisto). Explaining the bizarre hitherto unknown terrain proved quite an embarrassment to those who had to do it on live TV. Their usual impressive and esoteric technical jargon gave out and they had to resort to using such terms as "chevron", "race track", and "layer cake" to describe Miranda's unique features.

  It was initially thought that Miranda had been completely shattered and reassembled several times in its history, each time burying some parts of the original surface and exposing some of the interior. Now, however, a more mundane explanation involving the upwelling of partially melted ices seems to be in favor.

  Voyager 2 passed so close to Miranda and the light levels are so low there (almost 3 billion km from the Sun) that special measures had to be employed to avoid smearing the images. This was accomplished by rotating the entire spacecraft while the camera's shutter was open to compensate for its motion. The resulting images have the best resolution of the entire mission.

  It is actually possible to see Uranus's 4 largest moons with an amateur telescope, but Miranda is a real challenge. Perhaps with a very dark sky and a telescope with an 18 inch (50 cm) aperture or more it might be possible.

  Miranda 是天王星已知卫星中距其第十一近,也是天王星的大卫星中靠天王星最近的一颗。

  公转轨道: 距天王星129,850 千米

  卫星直径: 472 千米

  质量: 6.3e19 千克

  Miranda是莎士比亚的作品《暴风雨》中魔术师的一个女儿的名字

  它是由Kuiper于1948年发现。

  旅行者2号为了继续飞向海王星,不得不飞近天王星以获得推动力,由于整个飞行的方向几乎与黄道面成90度角,所以只与天卫五十分接近。在旅行者2号飞近之前,由于天卫五不是海王星的最大卫星,也没有什么特别之处,因此也不可能被选为主要研究对象,所以当时对于这颗卫星几乎是一无所知的。然而旅行者2号却证明了这是一颗非常有趣的卫星。

  天卫五是由冰与岩石各半混合而成。

  天卫五的表面是由众多的环形山地形和奇异的凹线、山谷和悬崖组成(其中的一座有5千米高)。

  起先,旅行者2号带来的天卫五图片上的情景使人们困惑不解。每个人过去都认为天王星的卫星的地质内部活动的历史极短(就像木卫四)。对那些进行现场直播的工作人员来说,如何去讲解这至今仍无法解释的古怪地形是一项很大的困难。他们常用的那些深奥难懂的行话也已经无济于事了,他们不得不用一些诸如“^或v的钜齿图”、“跑道”、和“多层蛋糕”之类的术语来措述天卫五的奇异的牲。

  后来人们认为天卫五自其产生后经历过多次的粉碎与重新聚合,并且每次都破坏了一部分的原始表面,露出一些内部物质。然而现在,另一更易被人们接受的理论产生了,那就是这些地形是由于熔化的冰而造成的。

  旅行者2号飞得离天卫五很近,又因为光线非常暗(离太阳大约30亿千米),所以必须采取特别措施来防止照片模糊。它在拍摄时不停地自转来跟上天卫五的运动。这些成功的照片是整个任务中分辨率最高的图片。

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